In a conventional four-stroke internal combustion engine, a power piston is disposed for reciprocating movement within a cylinder. The top of the cylinder is closed by a cylinder head that carries one or more induction poppet valves and one or more exhaust poppet valves. The induction poppet valve is timed so as to open as the power piston moves down the cylinder and, with the resultant partial vacuum, draws a combustible gas past the open poppet valve and into the cylinder. In respect of pressurised induction systems, the partial vacuum becomes positive pressure being forced into the negative pressure part of the cylinder as the piston moves down the cylinder. The induction poppet valve is then timed so as to close at the point when the piston is near the lowest point of its travel, thereby trapping a cylinder full of combustible gas. As the power piston is pushed back up the cylinder, by virtue of being connected to a crank that continues to rotate, it compresses the gas. At a point near the top of this cycle, called the compression stroke, a spark plug, which has been designed into the cylinder head, is sparked, causing the gas to ignite and rapidly expand as it explodes, pushing the piston down. As the piston comes back up again, the exhaust poppet valve (or valves) is (or are) timed to open, allowing the gases to escape.
Poppet valves have been used in internal combustion engines for many years, but display some disadvantages. Poppet valves are relatively expensive to manufacture and incorporate into cylinder heads of combustion chambers, due to the fine machining required to effect tolerances required for use of the valves in the hostile environment within the cylinder head.
Poppet valves, although fairly robust in construction, and although they initially create fluid-tight seals, restrict the flow of fuel and gases into and out of the engine, as the fuel and gas must flow around the valve and its associated stem. Poppet valves are also a source of vibration and noise through the effects of metal to metal contact with the cylinder head of the engine. Furthermore, as revolutions of the engine increase, the ability of poppet valves to open and close in time decreases in efficiency to the point where power output cannot increase further. Poppet valves are also a large source of friction, as is the camshaft and spring loaded follower generally used to open and close the valve.
There are known engines which do not comprise poppet valves, such as rotary engines and two-stroke piston engines, but such engines are generally inefficient in fuel consumption and costly to maintain.
It is therefore an aim of preferred embodiments of the present invention to overcome or mitigate a problem of the prior art, whether expressly mentioned hereinabove or not.